Tens of thousands of New Yorkers remained without electricity for a sixth day yesterday, with no end in sight to a power cut that the city's authorities were at a loss to explain.

Electrical engineers were drafted in from other states to try to return power to an estimated 80,000 people in Queens, New York's geographically largest borough. But the city's mayor, Michael Bloomberg, admitted that he had no idea how long it would take them. "It'll be done when it's done," he told reporters.

The outages began amid sweltering temperatures at the start of last week, affecting an area that included La Guardia airport. Dozens of flights were cancelled and security-screening machinery was brought to a halt before power to the airport was restored.

Consolidated Edison, New York's electricity provider, has been conducting manhole-by-manhole inspections of its cable network, but its task has been hampered in recent days by heavy thunderstorms that have flooded parts of the underground areas being examined.

"It was really a very extraordinary event, something that I've never seen before," said ConEd's chief executive, Kevin Burke, who did not talk to the media until five days into the power cut.

He admitted he was mystified about why high temperatures had knocked out the supply. "I don't know right now what has happened," he said.

ConEd originally estimated that only 2,500 of its "customers" - meaning anything from a single person in a flat to an entire apartment building - had been affected. But on Friday it revised its estimate to 25,000 customers, or up to 100,000 people. No more than 15% of those affected had so far had their power restored by Saturday, according to New York's office of emergency management.

"As soon as we get one feeder back online, another one burns out," one electrical worker told the New York Post. "We get power surges when something goes up, and that kills something else."

The emergency has stirred memories of the 2003 blackout in the north-east of the US, and the authorities' apparent paralysis has even prompted comparisons to Hurricane Katrina's aftermath.

Of Mr Bloomberg's public praise for ConEd's efforts, the New York Daily News said: "Not since President Bush stood with his arm around Fema [Federal Emergency Management Agency] chief Michael Brown post-Katrina and proclaimed, 'Brownie, you're doing a heck of a job,' has there been such a misplaced declaration of support." The mayor later conceded ConEd's performance was "intolerable".

One darkened bar in Queens was sardonically offering "ConEd special" drinks this weekend, but other business owners said they were facing financial crisis.